The ubiquitin-proteasome system (UPS) comprises one of the most important mechanisms for post-translational regulation of protein function in eukaryotic cells. The UPS comprises hundreds of enzymes that promote covalent attachment of ubiquitin and ubiquitin-like proteins (UBL) to target proteins, as well as enzymes that reverse the modification. Conjugation of ubiquitin to target proteins is a multi-step process (Weissman, 2001, Nat. Rev. Mol. Cell. Biol., 2:169-178; Finley, 2009, Annu. Rev. Biochem., 78:477-513; Schrader et al., 2009, Nat. Chem. Biol., 5:815-822; Deshaies et al., 2009, Annu. Rev. Biochem., 78: 399-434). The most intensively-studied consequence of ubiquitination is protein degradation. Given the importance of the UPS to regulatory biology there has been considerable interest in developing small molecule inhibitors as potential therapies for a range of human diseases. The UPS has been validated as an important target in cancer by clinical use of the proteasome inhibitor, bortezomib (Velcade), for the treatment of multiple myeloma and mantle cell lymphoma (Kane et al., 2003, Oncologist, 8:508-513; Colson et al., 2004, Clin. J. Oncol. Nurs.,8:473-480).
The AAA (ATPase associated with diverse cellular activities) ATPase p97 is conserved across all eukaryotes and is essential for life in budding yeast (Giaever et al., 2002, Nature, 418:387-391) and mice (Muller et al., 2007, Biochem. Biophys. Res. Commun., 354:459-465). p97 (also referred to as the transitional endoplasmic reticulum ATPase) is overexpressed in several cancers supporting the idea that it could be a target of general importance in oncology (Yamamoto et al., 2004, Clin. Cancer Res., 10:5558-5565; Yamamoto et al., 2003, J. Clin. Oncol., 21:447-452). Elevated expression levels of p97 have been associated with poor prognosis of cancer (Yamamoto et al., 2004, Ann. Surg. Oncol. 11:697-704; Tsujimoto et al., 2004, Clin. Cancer Res., 10:23007-3012). Additionally, p97 is an essential ATP hydrolase and thus, it should be druggable and have antiproliferative activity. Furthermore, p97 is essential for endoplasmic reticulum associated degradation (ERAD) (Ye et al., 2004, Nature, 429:841-847; Ye et al., 2003, J. Cell Biol., 162:71-84; Neuber et al., 2005, Nat. Cell Biol., 7:993-998). Blockade of ERAD is thought to be a key mechanism underlying the anti-cancer effects of bortezomib (Nawrocki et al., 2005, Cancer Res., 65:11510-11519). Given that p97 is implicated in ERAD, but otherwise has a more restricted role in the UPS compared to the proteasome, it is possible that drugs that target p97 might retain much of the efficacy of bortezomib but with less toxicity.